Previous methods for modifying the buoyancy of submerged objects have involved varying the volume of the object while holding the mass of the object constant or nearly constant. Pneumatically actuated ballast systems work in this way, as do mechanically actuated buoyancy pumps. Both of these methods achieve the result of altering the net density of the submerged object, which is equivalent to the displaced volume of the object divided by its mass. Additionally, dynamic lifting surfaces affixed to the submerged object have previously been used to control the position of the object relative to a towing platform. Dynamic lifting surfaces have the disadvantage of generating additional drag to produce lift which can counteract the buoyancy of the submerged object. They also require relative velocity between the submerged object and the submerging water, and hence do not work with the submerged object at rest.
The previously described methods create difficulties when implemented with small objects, such as small towed objects. Both pneumatically actuated and mechanically actuated volume-varying buoyancy systems can be impractical to implement on submerged objects having small dimensions, or which must operate at great depths, and thus are exposed to high external pressure. The previously described techniques may also be mechanically incompatible with certain towed objects, particularly small diameter sensor arrays. Other problems include use in environments in which it is impractical or problematic to store or generate the high pressure gas required by a pneumatic system.